1. Field of the Invention
The present invention relates to an exhaust gas purification device of an internal combustion engine having a NOx catalyst.
2. Description of Related Art
An occlusion reduction NOx catalyst (LNT) occludes NOx in a lean condition and discharges the NOx after reducing the NOx with HC or CO in a rich condition. If a NOx occlusion amount increases, NOx occlusion performance is deteriorated. If the NOx occlusion performance is saturated, the function as the NOx catalyst is lost. Therefore, fuel as a reducing agent is supplied to the NOx catalyst by making a rich condition periodically. Thus, the NOx occlusion amount within the NOx catalyst is eliminated by reducing and releasing the occluded NOx. This processing is generally called as rich purge control.
Accumulation of a sulfur component contained in the fuel degrades the NOx occlusion performance of the occlusion reduction NOx catalyst. When a large amount of the sulfur component accumulates, a state satisfying a sulfur release condition (temperature ≧600° C., air-fuel ratio ≦14.5) is made to release the sulfur component. This processing is generally called as recovery from sulfur poisoning. This processing is performed by estimating a degree of the degradation, for example, every 1000 km run. This processing causes fuel consumption aggravation and heat deterioration of a catalyst component because of elevated temperature. If the degradation degree of the NOx occlusion performance due to the accumulation of the sulfur component can be determined with sufficient accuracy, the recovery from sulfur poisoning can be performed when necessary. Accordingly, the frequency of performing the recovery from sulfur poisoning can be minimized. For this reason, an exact degradation determination technique of the NOx catalyst is desired.
For example, a method described in JP-A-2000-34946 compares a provable amount of the NOx occluded in the NOx catalyst (or amount indicative of its characteristic) at the time of start of the rich purge control with the amount of the NOx actually occluded (or amount indicative of its characteristic) in order to sense the performance degradation of the occlusion reduction NOx catalyst. The amount of the actually occluded NOx (actual NOx occlusion amount) is equivalent to the amount of the reducing agent consumed by the NOx catalyst while the rich purge control is performed once. Therefore, the actual NOx occlusion amount can be estimated by beforehand grasping a relationship between the fuel amount consumed as the reducing agent and the NOx amount, which can be reduced, through estimation of the fuel amount consumed as the reducing agent based on an air-fuel ratio sensed with an A/F sensor upstream of the NOx catalyst and an amount of fresh air (sensed with airflow meter or the like) supplied to the engine.
However, if the rich condition is made through combustion in a compression ignition internal combustion engine, the combustion becomes unstable in many cases. In such the cases, the HC component can vary or 1% or more of residual oxygen can be contained even in the rich condition. As a result, the output of the A/F sensor will shift. Since the fuel amount consumed in the reduction is estimated by using a signal of the A/F sensor, whose output has shifted, i.e., by using the air-fuel ratio information with low accuracy, an estimation error in the fuel amount consumed in the reduction enlarges. Accordingly, an estimation error of the actual NOx occlusion amount enlarges. As a result, accurate degradation determination of the NOx catalyst cannot be performed.
There is another method of obtaining the air-fuel ratio information. The method estimates the air-fuel ratio information based on the fuel injection amount calculated from an injection amount command value outputted to the injector and the fresh air amount. However, generally, the injector has a gain error and an offset error between a command injection amount corresponding to an injection amount command value and an actual injection amount. A variation in a period from an energization start to actual valve-opening of a nozzle is a component of the offset error, and a variation in a flow rate resistance of the nozzle is a component of the gain error. Therefore, it is difficult to estimate an exact air-fuel ratio from the fresh air amount measurement value and the injection amount command value. As a result, it is difficult to perform degradation determination of the NOx catalyst accurately.